In known processes malt is manufactured by treating cereal grains, for example barley, wheat, or rye grains, by a process comprising three stages; steeping, germinating, and kilning or drying. Grains are malted primarily to develop and/or activate enzyme systems such as amylase, which acts both to break down the starch of the cereal grains and to invest the malt product with the ability to convert starch to sugar.
During steeping the grains to be malted are vigorously aerated and stirred in water usually at temperatures in the range 50.degree.-60.degree. F. whereby the grains absorb water over a period of about two days from an initial water content of about 15% to a final water content of about 45%. During steeping the grains begin to respire very rapidly and break down their storage substances e.g. proteins and starch.
During germination, which often takes place over 5-7 days, the grains are converted to "green" malt. The germination conditions, particularly temperature, are controlled to minimise the development of roots and shoots from the grains, while still bringing about the desired enzymic changes in the cereal grains.
The prior art processes for steeping and germination are entirely applicable to the preparation of pale malt by the present invention, and will not be described in any further detail, being very well known to those skilled in the art.
Green malt is completely ready for processing for brewing purposes, except that it still lacks the typical malt aroma. Moreover, it cannot be stored without deterioration for an indefinite length of time. It is therefore subjected to kiln-drying.
Known processes for kiln-drying comprise a first step in which the green malt is withered, and a second step in which the withered malt is further dried. The withering of the green malt must be carried out quickly but carefully, since otherwise the saccharifying enzymes are damaged and there would be a marked deterioration in the quality of the malt. A sufficiently rapid dehydration is produced with an air throughput of 4,000 to 5,000 m.sup.3 /h and per ton. The temperature of the air introduced into the material to be withered should not exceed +60.degree. C. After approximately 10 hours' withering, the water content of the green malt has usually fallen to about 20%. Since, as is known, the saccharifying enzymes (amylases) in dry malt are much more stable to heat than those in fresh malt, the drying temperature can be raised in steps to 80.degree. and 85.degree. C. after the end of the withering process. The final water content of the finished malt should be about 4.5% and should be reached after 18 to 20 hours. After the withering process, i.e. during the kiln drying, saturation of the air with moisture no longer takes place on account of the hygroscopic properties of the malt, and therefore, in order to save energy, the air fed to the material being kiln dried during said further drying includes recycle air, i.e. waste air from the material being dried, as well as fresh air.
According to the teachings of the prior art, the recycling of air has been avoided during the withering step in the production of pale malt, i.e. before the water content of the malt has fallen to about 20%. It is known that the application of high kilning temperatures at high moisture contents increases the colour of the finished malt. For the production of pale malt therefore prior art processes have utilised dry air at temperatures not exceeding 60.degree. C during the withering stage, i.e. before the water content of the green malt has fallen to 20%. In the production of dark malt, however, recycle air has been used during the withering stage of the kilning in order to produce a dark malt colour.
Particularly in the withering stage, which is operated using full ventilation capacity, a relatively high energy consumption is necessary in order to heat the external air used in drying the material to the desired temperature. Another result of the high energy consumption is that the waste air flowing from the dried material should not, according to informed opinion valid up to now, be fed to the newly introduced fresh air as recycle air since the water content therein would only have a deleterious effect on the drying process.
It is desirable to reduce the energy consumption in kiln drying, particularly in the withering of green malt, which is required in treating the fresh air and which is relatively high in the known processes, especially during the colder seasons of the year.